Is Neil Gaiman's Sandman the greatest comic book of all time? In your dreams. Those who still consider Norman Mailer a major figure in American cultural life should know that he provided Sandman with a blurb from heaven. "Sandman," the Great Man asserted in a widely-circulated print-bite, "is a comic strip for intellectuals, and I say it's about time."

So caveat lector? After all, we have no way of knowing whether Mailer has read any comics since his days in the US Army over half a century ago. Successive generations of intellectuals have lavished praise on successive generations of comics: the allusive surrealism of George Herriman's Krazy Kat, the lacerating political satire of Walt Kelly's Pogo, the innovative graphic ingenuity of Will Eisner's Spirit, the sheer vitality of Stan Lee and Jack Kirby's original batch of Marvel Comics superheroes, and the scabrous confessionalism of Robert Crumb. More recently, Art Spiegelman's Holocaust fable Maus, Frank Miller's breathtaking redefinition of Batman, The Dark Knight Returns, and Alan Moore's devastating critique of the superhero, Watchmen, have all attracted attention far beyond the confines of the traditional comics ghetto.

However, nothing produced in the field during the past ten years has come close to the crucial combination of popular and critical success achieved by Gaiman's Sandman series. Uniquely among big-selling comics titles, this is a strip primarily concerned with ideas. Sandman is a story about story, a myth about myth, a postmodern metafiction with word balloons. "If this isn't literature," Peter Straub wrote defiantly in an afterword to the Sandman collection Brief Lives, "nothing is."

Between 1988 and 1996, the Sandman comic ran for 75 monthly issues - not to mention the odd special or spin-off - and was collected into ten "graphic novels". The last of them, The Wake, is published this week. By the time the title was wound down - at the author's behest - Sandman had accumulated a formidable shelf of awards, was selling over a million copies a year, and had a Who's Who of fantasy and horror writers (Stephen King, Gene Wolfe, Clive Barker, Ramsey Campbell) queuing up to sing its praises.

Yet Sandman's beginnings were inauspicious. In a comics world dominated by fights in tights, the primary assets are titles and characters rather than artists or writers. Editors seek ways to revamp obscure or faltering characters. The original Sandman was a 1940s no-hoper disinterred from the mouldering pages of Justice Society Of America: a Bruce Wayne-like millionaire playboy in trenchcoat, fedora and mask, who fought crime by putting villains to sleep with a gas-gun. Unfortunately, he had an identical effect on readers.

DC Comics editor Karin Berger therefore risked little when she handed the poor schlub over to Neil Gaiman (a personable English pop-culture hack) for updating. Gaiman repaid her percipience: in a major coup, he transformed the dullest excuse for a superhero ever to waste woodpulp into the dread figure of Morpheus, Lord Of Dreams - the Prince of Stories himself.

Gaiman's Sandman was nothing less than a personification of the human imagination. His saga was described by one critic as "a secret history of the unconscious". Morpheus - aka Dream, or Oneiros - is one of the archetypes who call themselves The Endless, the others being Destiny, Despair, Desire, Delirium (who used to be Delight) and Destruction (who retired in the 17th century because humans no longer needed him). Dream is a tall, pallid figure with a shock of black hair, somewhat resembling The Cure's Robert Smith, minus the lipstick, after being thoroughly stretched on a rack. His realm is The Dreaming, an ever-changing Gormenghast-like castle surrounded by nebulous landscapes; it's where we go when we dream.

Morpheus has absolutely no sense of humour. Fortunately, his elder sister Death (who nevertheless looks younger) does. An adorable punkette with an irredeemably optimistic outlook, she incongruously became many readers' favourite. Her solo story, Death: The High Cost Of Living, will be filmed with a screenplay by the author.

Gaiman is an omnivorous reader and a formidable researcher. Sandman is packed with pastiche, allusion and greater and lesser arcana of all descriptions. Historical figures such as William Shakespeare, who appears as himself here in two key tales based on A Midsummer Night's Dream and the Tempest, rub shoulders with the fictional creations of others, some derived from the repertory company of DC's own comics; others from the entire tapestry of human myth.

The many hands who have illustrated the strip over the past eight years have borrowed the appearances of a variety of notables. Lucifer, the Fallen Angel, is the young David Bowie; Delirium resembles the avant-garde novelist Kathy Acker, and Fiddlers' Green, a place in The Dreaming which decided to get up and walk, does so as G K Chesterton.

The most consciously literary mass-market comic strip ever, Sandman finally counts as a conditional triumph. As is appropriate for a dream about dreams, Sandman is gossamer-thin, not always able to support the weight of the symbolic anvils Gaiman places upon it. The strip rarely stops winking at the reader, and even at its grimmest and goriest it remains oversweet. Gaiman's weakness remains the cutesiness demonstrated in his overblown novel and TV series Neverwhere. But even if Sandman isn't the greatest comic of the century, it is good enough to demand to be judged by those standards.

Before Sandman, the Lord of Dreams had no dreams of his own: the Prince of Stories lacked a tale. Neil Gaiman has remedied that. The Wake is a sober, sombre conclusion - literally, a dying fall - to a sequence that undeniably constitutes a major achievement. There is nothing quite like it anywhere else.